Embodiments of the invention relate generally to thermal energy storage and, more particularly, to a system and method of storing thermal energy in an underwater storage device.
Renewable energy (RE) sources offer an alternative to conventional power sources in an age of dwindling non-renewable energy sources and high carbon emissions. However, RE sources are often not fully exploited because many forms of renewable energy are not available when the peak demand is present. For instance, RE sources may be most available during undesirable off-peak hours, or may be located in areas that are remote from population centers or locations where power is most needed, having to share the grid during peak hours along with all the other peak power sources.
RE sources may include hydro power, geothermal, Ocean Thermal Energy Conversion (OTEC), as examples. Hydro power, for instance, when combined with a reservoir is one RE source that can be throttled up and down to match or load-follow the varying power loads. Geothermal and OTEC are also good baseload RE resources; however, viable locations for geothermal and OTEC are limited. It is to be understood that an ocean thermal energy converter, while traditionally utilized across the thermocline of an ocean, can additionally apply to fresh bodies of water that have a temperature difference between surface water and deep water. RE sources may also include solar, wind, wave, and tidal, as examples. However these sources tend to be intermittent in their ability to provide power. Energy storage is thus desired for those sources to substantially contribute to the grid energy supply.
Cost-effective storage for the electrical grid has been sought from the beginning of electrical service delivery but is not yet available. In the absence of affordable storage, the variation in power demand throughout a day, and season-to-season, requires generation assets that are utilized only part of the time, which can increase capital, operations, and maintenance costs for assets used at less than full capacity. Also, some generation assets are difficult to throttle or shut down and are difficult to quickly return to full power. Energy storage can provide a buffer to better match power demand and supply allowing power sources to operate at higher capacity and thus higher efficiency.
Compressed air energy storage (CAES) is an attractive energy storage technology that overcomes many drawbacks of known energy storage technologies. One approach for CAES is illustrated in FIG. 1. CAES system 10 includes an input power 12 which can be, for example, from a renewable energy source such as wind power, wave power (e.g., via a “Salter Duck”), current power, tidal power, or solar power, as examples. In another embodiment, input power 12 may be from an electrical power grid. In the case of a renewable energy (RE) source, such a source may provide intermittent power. In the case of an electrical power grid, system 10 may be connected thereto and controlled in a fashion that electrical power may be drawn and stored as compressed fluid energy during off-peak hours such as during late evening or early morning hours, and then recovered during peak hours when energy drawn from system 10 may be sold at a premium (i.e., electrical energy arbitrage), or to augment base load power systems such as coal to provide peaking capability by storing inexpensive base load power.
Input power 12 is coupled to mechanical power 14 to compress fluid from a fluid inlet 16, and fluid compression 18 results. Cooling may be introduced via pumps and heat exchangers or through direct contact between the compressed fluid and a cooling fluid. Fluid from fluid compression 18 is conveyed to compressed fluid storage 20 via a fluid input 22.
When it is desirable to draw stored energy from system 10, compressed fluid may be drawn from compressed fluid storage 20 via fluid output 24, and fluid expansion 26 occurs, which results in available energy that may be conveyed to, for instance, a mechanical device that extracts mechanical power 28 for electrical power generation 30. The generated electrical power may be conveyed to a grid or other device where it is desirable to have electrical power delivered. Outlet fluid 32 is expelled to the environment at generally standard or ambient pressure.
When operated close to isothermally (i.e., quasi-isothermally), system 10 includes forced-convection cooling 34 to cool the fluid from fluid compression 18 and forced-convection heating 36 to heat the fluid from fluid expansion 26. Because compressed fluid storage occurs at generally ambient temperature and pressure, both cooling 34 for fluid compression 18 and heating 36 after fluid expansion 26 may be performed using the vast amount of environmental fluid that surrounds system 10 at ambient temperature and pressure.
FIG. 2 illustrates a marine-based, quasi-isothermal implementation of CAES system 10. Components of system 10 are positioned on a platform 38 proximately to the water surface of a sea 40. Platform 38 is supported by the seafloor 42. A compressed air storage assembly 44 is positioned at an average depth 46, and a compressor/expander system (C/E) 48 is coupled to a generator 50. C/E 48 may include multiple stages of compression and expansion for quasi-isothermal operation, and a heat exchanger package (not shown in this figure) may cool or reheat the fluid between the stages of compression or expansion, respectively.
A fluid hose or pipe, or pressurized-fluid conveyance system 52 connects fluid storage bag assembly 44 with the C/E 48 at or near the surface of sea 40. When power is input 54 to C/E 48, C/E 48 operates to compress fluid, convey it to fluid storage tube assembly 44 via fluid hose or pipe 52, and store the energy therein. Power 54 may be provided via a renewable source such as wind, wave motion, tidal motion, or may be provided via the generator 50 operated as a motor which may draw energy from, for instance, a power grid. Also, C/E 48 may be operated in reverse by drawing compressed stored energy from fluid storage tube assembly 44 via fluid hose or pipe 52 to drive the generator 50 to generate AC or DC power.
While operation of CAES system 10 in a marine-based quasi-isothermal operation takes advantage of the generation of energy from cost-effective sources, quasi-isothermal CAES systems typically compress fluid in a plurality of compression stages, and with cooling or heating within or between stages achieved via pumps and heat exchangers. An adiabatic CAES system, however, allows for storing thermal energy generated during fluid compression, which is not disposed of but used subsequently to preheat the compressed air prior to or during fluid expansion.
If there are enough compression stages, the system can operate at close to isothermal efficiency simply by exchange enough heat with the external environment. However, a compression system with a large number of stages may be quite expensive.
In newer, adiabatic CAES designs, thermal energy is stored at high temperatures that require expensive media and containment systems. For example, one proposal for thermal storage includes the use of thermal storage containers filled with stone or ceramic bricks, stored at 600° C. Such a high temperature system is challenging and expensive to enclose and insulate. Water, on the other hand, has very high heat capacity, is very inexpensive, but is challenging to use as a storage medium because of its relatively low boiling point at low or moderate pressures.
It would be advantageous to have thermal energy storage systems that incorporate water or other low cost, non-toxic liquids as the energy storage medium. Given that thermal energy storage systems can be deployed both on land and offshore, or partly on land and partly offshore, it would be advantageous to have thermal energy storage systems which can incorporate low cost thermal storage in both environments.